The invention relates to a turbine blade having a bracket arranged in the tip region of the blade and projecting beyond the blade profile.
In particular in smaller turbomachines, the gap losses occurring in the tip region of the turbine blades, i.e. between the blade tip and the blade shroud, may assume considerable proportions and thus may lead to a distinct loss of efficiency. These gap losses are caused by the pressure difference between the pressure side and the suction side in the tip region of the blade, as a result of which there is an overflow of the working fluid. This overflow can be reduced by increasing the friction losses between the blade tip and the blade shroud, a factor which leads to a reduction in the gap losses and thus to an increase in the efficiency.
A known measure aimed at reducing the gap losses and increasing the reliability of the turbine blade is the arrangement of a rib or bracket (so-called minishroud or winglet) projecting laterally beyond the blade profile in the tip region of the blade (xc3x9c. Okapuu, presentation xe2x80x9cAerodynamic design of first stage turbines for small aero enginesxe2x80x9d, held as part of the lecture series 1987-07 of the Karman Institute for Fluid Dynamics, on the topic xe2x80x9cSmall High Pressure Ratio Turbinesxe2x80x9d, Jun. 15-18, 1987, pages 1-4 and FIGS. 1-4). GB-A-2 153 447 and GB-A-2 050 530 also disclose blade bodies having brackets projecting onto the suction side and onto the pressure side. GB-A-2 050 530 also indicates a blade body having a bracket which is arranged on the suction side and above which the blade body rises on all sides.
However, all these solutions result in an increase in both the leading and trailing wedge angles of the blade and thus in corresponding efficiency losses.
The object of the invention, in attempting to avoid all these disadvantages, is to achieve a further improvement in efficiency for a turbine blade which is provided with a bracket arranged in the tip region of the blade and projecting beyond the blade profile.
According to the invention, this is achieved in that, the bracket is formed on the pressure side of the blade and only in a section of a defined surface region of this pressure side. In this case, this defined surface region constitutes that region of the pressure side which is enclosed by an imaginary chord bearing against the pressure side of the blade body both in the region of the leading edge and in the region of the trailing edge.
The pressure-side section having the bracket is formed between and at a distance from two points lying on the pressure side of the blade body, the first point being arranged in the region of the leading edge and the second point being arranged in the region of the trailing edge, and the imaginary chord bearing against both points. It has proved to be especially expedient to form a distance a1, a2 between the first point and the bracket and between the second point and the bracket, and this distance a1, a2 in each case corresponds approximately to a blade thickness d1, d2 in the corresponding region of the blade body.
Since the bracket therefore extends only over part of the pressure side, both wedge angles of the blade body, i.e. the leading angle and the trailing angle, can be kept small. The wedge angles can be reduced further through selection of the defined distances a1, a2. This results in an acute blade geometry, which ensures that high Mach numbers in the leading-edge region and wake zones in the trailing-edge region of the blade body are avoided. Ultimately, a further improvement in efficiency is achieved while a reduction in the gap losses continues to be ensured.
Starting from the leading edge, the bracket has a maximum height hmax above the pressure side in a region of 30 to 40% of the chord length l of the blade body. In this case, the height of the bracket continuously decreases both in the direction of the leading edge and in the direction of the trailing edge of the blade body. The maximum height hmax of the bracket corresponds approximately to a blade thickness d3 as formed in a region of the blade body which is adjacent to the bracket and remote from the blade tip. Furthermore, it is advantageous if the bracket is of wavelike design.
It has been found that, in particular with such a design of the bracket, optimum flow guidance can be achieved.